A lithium ion battery is a secondary battery having a structure, in which, at the time of charging, lithium begins to dissolve as ions from a positive electrode and moves to a negative electrode to be stored therein, while, at the time of discharging, on the contrary, the lithium ions return from the negative electrode to the positive electrode. Since the lithium ion battery has high energy density, and a long life cycle, it is widely used as a power supply for home appliances such as a video camera, portable electronic equipment such as a notebook computer and a portable telephone, and electrically-drive tools such as a power tool. Recently, the lithium ion battery is also applied to a large-sized battery that is mounted in an electric vehicle (EV), a hybrid electric vehicle (HEV), or the like.
This kind of a lithium ion battery is constituted of a positive electrode, a negative electrode, and an ion conducting layer inserted between both of the electrodes. As the relevant ion conducting layer, a separator constituted of a porous film, such as polyethylene and polypropylene, which is filled with a non-aqueous electrolytic solution, is generally used. However, since such an organic electrolytic solution using a flammable organic solvent as a solvent is used as an electrolyte, it is required to improve the structure and material for preventing volatilization or leakage, and also, it is required to install a safety device for suppressing an increase in temperature at the time of a short circuit and to improve the structure and material for preventing a short circuit.
In contrast, an all-solid lithium battery that is constituted by solidifying the whole battery using a solid electrolyte using lithium sulfide (Li2S) as a starting material does not use a flammable organic solvent, and thus, it is possible to attempt the simplification of a safety device. In addition, the battery can be made excellent in the production cost and productivity. Furthermore, it is possible to attempt high voltage by laminating it in a cell in series. Furthermore, for this kind of a solid electrolyte, a Li ion only moves, and thus, there are no side reactions by the movement of anion. Therefore, it is expected that it leads to improve safety or durability.
Such a solid electrolyte that is used for a battery has high ionic conductivity and should be chemically or electrochemically stable, and for example, lithium halide, lithium nitride, lithium oxoate, or derivatives thereof are known as the candidate of raw material.
With regard to this kind of a solid electrolyte, for example, Patent Document JP 3184517 B1 discloses a sulfide-based solid electrolyte, in which a high temperature lithium ion conductive compound composed of lithium phosphate (Li3PO4) allows to be in lithium ion conductive sulfide glass represented by General Formula, Li2S-X (wherein X represents at least one sulfide of SiS2, GeS2, and B2S3).
In addition, Patent Document JP 3744665 B1 discloses a sulfide-based solid electrolyte including a lithium ion conductive material that is a composite compound represented by General Formula, Li2S—GeS2—X (wherein X represents at least one type of Ga2S3 and ZnS) as a material that is crystalline and exhibits very high ionic conductivity, that is, 6.49×10−5 S·cm−1 of the ionic conductivity at room temperature.
Patent Document JP 2001-250580 A discloses lithium ion conductive sulfide ceramics that are sulfide ceramics having high lithium ionic conductivity and decomposition voltage, which have Li2S and P2S5 as a main component, and the composition of Li2S=82.5 to 92.5 and P2S5=7.5 to 17.5 by mol %, and among them, preferably the composition of Li2S/P2S5=7 (Compositional Formula: Li7PS6) in a mole ratio.
Patent Document JP 2011-96630 A discloses a lithium ion conductive material that is represented by Chemical Formula: Li+(12-n-x)Bn+X2−(6-x)Y−x (Bn+ is at least one type selected from P, As, Ge, Ga, Sb, Si, Sn, Al, In, Ti, V, Nb, and Ta, X2− is at least one type selected from S, Se, and Te, Y− is at least one type selected from F, Cl, Br, I, CN, OCN, SCN, and N3, and 0≦x≦2), and has an argyrodite-type crystal structure.
Patent Document JP 2010-540396 A discloses a lithium argyrodite, which is a solid compound capable of being prepared as a single layer in addition to high fluidity of the lithium ion, and is represented by General Formula (I) Li+(12-n-x)Bn+X2−6-xY−x, in which in Formula, Bn+ is selected from the group consisting of P, As, Ge, Ga, Sb, Si, Sn, Al, In, Ti, V, Nb, and Ta, X2− is selected from the group consisting of S, Se, and Te, Y− is selected from the group consisting of Cl, Br, I, F, CN, OCN, SCN, and N3, and 0≦x≦2.
The present inventor focused on a compound having a cubic crystal structure belonging to a space group F-43m and being represented by Compositional Formula: Li7-xPS6-xHax (Ha is Cl or Br), as a solid electrolyte material used for a lithium ion battery.
However, when such a compound is used as a solid electrolyte for a lithium ion battery, there are problems in that electron conductivity is high, and charge/discharge efficiency and cycle characteristic are not increased as predicated.
Therefore, an object of the invention is to provide a novel sulfide-based solid electrolyte for a lithium ion battery, in which for a compound having a cubic crystal structure for a space group F-43m, and being represented by Compositional Formula: Li7-xPS6-xHax (Ha is Cl or Br), the charge/discharge efficiency and cycle characteristics thereof can be increased by reducing the electron conductivity through increasing the lithium ion conductivity for the compound.